1. Technical Field
The present disclosure relates generally to a method and apparatus for interference suppression in radio-over-fiber (RoF) communication systems.
2. Related Art
The radio-over-fiber (RoF) technology is one of the important technologies in high-speed communication systems. In a RoF system, a base station (BS) is located in a head end unit (HEU) to transmit and receive signals to/from a mobile station (MS) through a plurality of remote antenna units (RAU) geographically distributed. These RAUs connect the base station via optical fibers with unequal lengths. In the orthogonal frequency division multiplexing (OFDM) communication system having a fixed moving trajectory (such as railways), the RoF technology may extend the coverage of the base station for reducing implementation and maintenance cost and improving the mobile station signal quality. In the RoF system, the base station needs to control the plurality of RAUs at different positions. When the mobile station moves among these RAUs, the delay spread caused by different lengths of optical fibers may exceed the processing range of the mobile station transceiver, thus a decline of the signal quality may occur, or the delay spread of signal may become too large and lead to disconnection of mobile stations.
There are several methods. For example, in one method a time delay module is designed in the baseband or the radio frequency (RF) end to perform estimation and compensation for the time difference. For example, in a RoF system 100 of FIG. 1, L1 and L2 represent the required fiber lengths that the base station 110 reaches the RAU1 and the RAU2, respectively; τ1 and τ2 represent signal propagation times of RAU1 with transmit power γ1 and RAU2 with transmit power γ2, respectively. The fiber of unequal lengths may produce additional delay(s). For example, in FIG. 1, D represents the distance between the RAU1 and the RAU2, the distance between the RAU1 and the mobile station 120 approximates d, and the distance between the RAU1 and the mobile station 120 approximates D−d. Thus an additional delay Δτ (i.e. |τ2−τ1|) is estimated to approximate |L1−L2|/Vf, where Vf is the signal propagation speed in the optical fiber. In another method a time delay unit is used in the base station end to delay uplink/downlink signals, and a time delay module is used to measure and generate a time delay control signal, and then a time delay compensator is used to compensate the signal in the optical fiber according to time delay control signal, to maintain synchronization between the base station and transmit signals of the RAU. When the time delay module is designed at the baseband, this method may also change the design of the base station.
In another method, it is proposed to pad each optical fiber length between head end and each RAU to result in a same optical fiber length for reaching these RAUs. FIG. 2A shows curves illustrating the impact of the fiber length on the throughput, wherein a solid curve 212 represents the throughput with equal fiber length and a dotted curve 214 represents the throughput with unequal fiber length. FIG. 2B shows a schematic view illustrating the comparison of optical fiber length versus the carrier to interference and noise ratio (CINR), wherein a solid curve 224 represents the CINR with equal fiber length and a dashed curve 222 represents the CINR with unequal fiber length.
Yet in another method the length of the guard interval (GI) or the time gap in the OFDM system is adjusted. For example, some techniques provide the relationship of the GI length and the delay spread caused by unequal optical fiber lengths. In other words, the GI length is at least greater than three times of the maximum delay spread. To support a greater fiber length difference, the approach of increasing the GI length may be used, such as shown in FIG. 3, a part 310 of an original available spectrum is copied into a GI 320.
Among the existing RAU control technologies, some may need to change the base station design or are coupled with high complexity to introduce distortion; Some technologies of fiber equalization lead to high construction cost, or lacking flexibility of adjustment; Some GI adjustment technologies sacrifice the spectral efficiency, and spend more resources for transmitting duplicate signals. Therefore, it may need to design an interference suppression technology in the RoF communication systems to determine whether multipath mode may cause disconnection, and automatically adjust the RAU transmission power, with features including adjustability, simple structure, and unaffected spectrum efficiency.